Method and device for low energy consumption or granular products or the like containing moisture fixed or deposited at the surface with a constant yield

ABSTRACT

A hot drying agent discharged from the section for material transport of the bound moisture, having a low moisture content, is recirculated to the front part of the drying area; the moisture content of both of the fed and discharged material is measured and by means of the differential-analogue signal thus obtained the stream of the material to be dried and in a given case the heating of the drying agent is controlled. An apparatus for performing the drying operation is also provided.

FIELD OF APPLICATION OF THE INVENTION

The invention relates to a process for drying agricultural and othergranular products or produces with surface or bound moisture content,with a constant output and an energy saving method, in course of whichthe material to be dried is brought into contact with a heat carryingdrying agent, while advancing through the drying area; thereafter theheat content of the drying agent laden with vapour is either partlyutilized or led directly into the atmosphere, while the dry material,having been freed of its moisture content to the predetermined extent,is led to further processing or use.

Furtheron the invention relates to an equipment being suitable toperform the aforesaid process, having a drying area provided with a feedand discharge opening for the material to be dried, with a duct throughwhich the pressure fan delivering the drying agent is connected and in agiven case with an exhaustor for the suction of the drying agent, beingconnected via an exhaust duct, while before or behind the pressure fan aheating equipment for heating the drying agent is inserted.

STATE OF THE ART

It is a well known fact that dryers expel the liquids from materialscontaining the moisture on the surface or in bound form by evaporation.The thermal energy required for evaporation is led to the material to bedried either by radiation, conduction or convection accordingly,directly or by the intervention of some heat carrying agent, beinggenerally a gaseous substance. The vapour having been evaporated isdelivered by the heat carrying agent itself into the atmosphere. Thedried material being freed of its moisture content to the predeterminedextent is led on an other way to use or further processing.

In course of the drying process it is of utmost importance that as wellthe control of the drying process as the design of the dryer shouldfacilitate that the energy required for evaporation should reach thematerial to be dried with a high efficiency and uniformly; further thatduring of the process losses should be minimized. Losses result in thefirst place from the heat escaping through the surface of the equipment,secondly from the heat leaving with the dried material, thirdly with themoisture converted into vapour and with the heat content of the vapourcarrying drying agent.

Although with the processes and equipments having been realized inpractice, the mentioned requirements were mostly met, but due to thepreviously prevailing relative energy abundance and the low price ofenergy carriers, less attention was paid to the economy of the dryingprocess. Due to this fact, direct intervention to the drying process, ase.g. continuous control of the output, coordination of heating energyconsumption and output, utilization of the heat content of the exitingagents were mostly neglected. As a consequence, it often occurs that thequality of the dryed product is not uniform, furtheron much more energyis used as theoretically needed.

SUMMARY OF THE INVENTION

The task of the invention consists of elimination of thesedisadvantageous features, simultaneously to ensure the uniform qualityof the dried product.

The invention is based on a physical phenomenon, that far the intensityof the expulsion of moisture from certain materials to be dried byevaporation is considerably changing during the drying process. On thefirst interval, the moist material is only preheated, as in equipmentoperating mostly with atmospherical pressure, heating up to at about60°-120° C. is needed to start rapid evaporation of the water. When theliquid contained and the material have reached the temperature ofintensive evaporation, discharge of the moisture bound to the materialsurface or in the vicinity thereof begins. Here, if the structuraldesign of the dryer enables an intensive heat supply, a considerablequantity of moisture can be expelled in a relatively short time, and ina nearly steady manner. To expel the moisture from the inside of thematerial, i.e. separation of the so-called bound moisture represents aconsiderably slower process. Heat energy transport begins through thematerial surface inwards, and at the same time moisture transport in theopposite direction, i.e. outwards takes place. The effect may beobserved by the fact that the speed of moisture expulsion decreases, thetemperature of the material increases, while the heat carrying gas isleaving the system with a decreased moisture content, however, at anincreasing temperature. The conditions, e.g. for drying corns of maize,are illustrated in FIG. 1.

During the total drying period of about 4.5 hours, about 1/3 hour isrequired for heating up, the phase of intensive drying withapproximately constant speed lasts about 1 hour, while the remainingtime /4.5-1.3=3.2 hours/ represents the period needed for the previouslydescribed transport of heat and counter directed material transport.

Based on aforesaid, the task set was solved by the invention in such away that the hot drying agent with a low degree of humidity dischargedfrom the drying section where material transport of the bound moistureis taking place, is recirculated to the beginning of preheating andintensive drying. Simultaneously all other heating of these sections isstopped. Hereby the considerable heatloss occuring at the discharge fromthe section where material transport, i.e. removal of the bound moistureis taking place, will be regained without the use of a separate heatregenerating equipment. As a consequence, the only loss will appear atthe remaining heat content of the drying agent, leaving the preheatingand intensive drying sections, respectively.

As the drying agent is almost saturated when leaving the equipment, thelosses are very low.

The basic idea of the invention involves also the recognition thatreclaim of a considerable part of the perceptible heat leaving with thedried material without having been utilized, can reasonably be combinedwith the solution previously mentioned, in particular with air-dryers.When e.g. atmospherical air is allowed to stream, expediently via aventilator, through the stream of the discharging dry material,temperature of the dry material will approach that of the ambient air toan extent having been defined by the conditions of heat-transfer, whilethe part of the heat content determined by cooling is transferred to theair current. The air current, being warmer than the environment,absolute moisture content of which equals to that of the ambient air,can be well utilized in the drying process in such a way that it issimply admixed to the hot air current entering into the section wherematerial transport of the bound moisture is taking place, or it is ledas combustion air into the firing area of the dryer.

By using the process according to the invention, both disadvantageousconditions having been mentioned in the preamble, namely the heat lossesarising from discharging the dried material and from emitting the dryingagent, may be considerably reduced. In accordance with the inventionthese quantities of heat can be utilized to a high extent to cover thethermal need in the preheating section and in the section for intensivedrying, respectively. Accordingly, the only heat loss will appear in thequantity leaving therefrom. The appropriate numerical values will bedetailed later in connection with an example relating to the process.

It is a well known fact that e.g. when drying vegetable products, themoisture content of the material to be dried varies within rather widelimits, resulting with all types of dryers in a variable moisturecontent of the final product, consequently, either the quantity of thedrying agent, or the temperature of the drying process is to be changed,i.e. the intensity of the heat source has to be modified.

Uniformity of the quality of the dried product is ensured according tothe invention in such a way that moisture content of the moist materialto be fed into the drying area and that of the dry material dischargedtherefrom are determined by measurement, and by means of adifferential-analogue signal thus obtained, the flow of the material tobe dried, in some cases the heating of the drying agent is controlled.

With the knowledge of the parameter of the produce contained in thedryer, the quantity of the water to be expelled can be determined. Withthe system according to the invention, the flow of material shouldprimarily be modulated in order to obtain a time wise constant expulsionof the moisture. In case of gravimetric material flow, said processinvolves the considerable advantage, in so far as quantitative controlof the energy carrier /oil, gas, steam/ becomes practically superfluous,accordingly, heating equipment may be of a quite simple design. So, e.g.with the existing numerous tower-dryers, when using the processaccording to the invention, there is no need to exchange the firingequipment, no essential reconstruction is required. By controlling acommon slide valve, having been inserted into the path of thegravimetric material flow, by the previously describeddifferential-analogue signal, the task set can be easily solved. Heatingrequirement of a drying equipment being operated in the described way,theoretically varies only in dependence of the change in temperature ofthe ambient air.

The equipment according to the invention serves for the performance ofthe process according to the invention, which can be developed by thesimple reconstruction of the already existing equipments described inthe preamble, mainly gravimetric tower-dryers. Characteristic feature ofthe equipment consists of the drying area subdivided into sections forpreheating and for intensive drying, into a section where materialtransport, i.e. transport of the bound moisture takes place, and into acooling section; the sections for preheating and intensive drying areconnected to the section for the transport of material, i.e. the boundmoisture by a duct, further hygrometers are installed at the inlet forthe moist material and in the vicinity of the discharge opening, thedifferential-analogue signals of the hygrometers are led through thetransmitter to the control-unit regulating the intensity of materialdischarge.

In an advantageous form of construction the unit controlling theintensity of output is provided with a common slide valve which isarranged at the discharge opening, while the signals of the hygrometersare fed, e.g., through a transmitter to the magnetic valves of theworking cylinder actuating the mentioned slide valve.

Based on theoretical examinations and on experience it is possible toadjust the proportional relations of the drying sections, thus enablingadjustment to various sorts and a variety of moisture contents of thematerial to be dried. In particular cases or in case of delicateproducts it may seem expedient, if the section for preheating andintensive drying and the section for the material transport of the boundmoisture are separated by a partition plate, which can be displaced independence of the desired proportion between the sections, and thehygrometer inserted at the input of the moist material is connectedthrough a transmitter to the actuating unit of the partition plate. Thesignals of the hygrometers may be led into the memory of themicroprocessor control unit.

In an advantageous form of construction before or after the pressure fanan oil or gas burner of the heating equipment is installed, andcontrolled by a heat sensing element arranged in the input duct of thepressure fan.

The delivery side of the cooling ventilator of the cooling section isconnected expediently into the firing area of the oil or gas burner.

In case, e.g. a heat exchanger is applied as a heating equipment, it isexpedient to connect the delivery side of the cooling ventilator of thecooling section to the input duct of the pressure fan for the dryingagent.

In certain cases it might be advantagous if the equipment is providedwith an exhaustor for the drying agent, and the output duct thereof isconnected to an output bonnet or tray adjoinning with the section forpreheating and intensive drying.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in details by means of an advantageousform of construction serving as an example, by the aid of theaccompanying drawings, wherein

FIG. 1 is showing the drying curve for grains of corn, with theindication of each section in the drying process;

FIG. 2 is the conceptual design of an advantageous form of construction.

EMBODIMENTS

Referring to FIG. 1, it appears clearly that out of the total time need,amounting to about 4.5 hours, approximately 1/3 hour, i.e. twentyminutes, are required for preheating the grains, /Section I/. The nextsection serves for the intensive drying with an approximately constantspeed, requiring about 1 hour /Section II/, the remaining time,amounting to about 3.2 hours, represents the period needed for thematerial removal of the bound moisture /Section III/. From the left-sideordinate belonging to curve A it is seen that in sections I and II i.e.in 29% of the total drying time, 68.8% of the moisture is leaving, whilein the remaining 71% of the time, only 31.2% of the moisture can beexpelled.

On the right-side ordinate of curve B development of the surfacetemperature of the grains can be seen at a temperature of 120° C. of thedrying air. The dashed lines are showing the respective temperaturevalues belonging to the different initial moisture contents.

FIG. 2 is showing the conceptual design of a tower-dryer foragricultural grains having been constructed in accordance with theinvention.

The material to be dried, in our example the corn, is entering throughthe inlet 1 for the moist material. Moisture content of same is sensedby the hygrometer 2. The value measured is fed into the memory of thecontrolling microprocessor, where it remains and electronically can berecalled whenever needed in course of the drying process. The grains ofcorn are continuously passing through sections I and II, wherepreheating and intensive drying with a constant speed is taking place,as it can be seen in FIG. 1. With the knowledge of the parameter of thematerial subject to the drying process, modulation depth of the materialflow can be determined, e.g. by means of a microprocessor controldevice, by the aid of mathematical formulae and diagrams obtainedempirically for the dryer. Based on the signal of the hygrometer 3having been arranged at the discharge opening 8, the controlcharacteristics obtained in accordance with the empirical diagramsshould be corrected in case of a difference. After having amplified thesignal thus obtained in transmitter T, it serves as a pulse to controlthe magnetic valves 4 and 5, which valves are controlling the input ofe.g. the hydraulic oil into the working cylinder 6 to an extent definedby the differential-analogue signal. Consequently, the common slidevalve 7 is opening and closing the discharge opening 8 in compliancewith the predetermined moisture content of the discharged grains. Inthis way the material flow in the dryer is automatically set, whichensures optimal operation of the complete equipment.

With a considerably varying moisture content of the material to bedried, the position of the partition plate 12 can be changed, wherebythe limits of sections II and III are also modified.

Delivery side of the cooling ventilator 9 of the cooling section H isconnected to the input duct 17 of the pressure fan 10. Output of the oilburner 18 is controlled by the heat sensing element 11 arranged in theinput duct 17 of the pressure fan 10. Since the extent of heatextraction in the cooling section H is slightly affected by the massflow-of the material to be dried, it is essentially influenced only bythe temperature of the ambient air. The temperature of the drying agententering into section III, which can be waste gas-cooling air or ambientair, is kept by the heat sensing element 11 practically at a constantlevel. The drying agent is streaming into the system on the surfacebetween the partition plate 12 and the cooling section H. By means ofthe duct 13 the drying agent leaving section III is recirculated to thesections I and II. From the outgoing bonnet 14 the duct 15 leads thecooled and vapour-laden drying agent to the exhaustor 16, from where itflows into the atmosphere. In case of necessity, a waste catching deviceand discharging means may be inserted, which are not illustrated. Incourse of the drying process dust and/or other substances, solidmaterial particles become detached from the material to be dried,collecting and removal thereof can be effected by using already knownelements.

The position of the partition plate 12 is changed in dependence of thesignal coming from the hygrometer 2. With decreased moisture content,the sections I and II are shortened, with increasing moisture contentthe sections I and II become longer. The correlation between themoisture content and the length of the sections is depending on theproperties of the material to be dried, and are to be determined foreach material by measurement, /see FIG. 1/.

Beside the form of construction illustrated the equipment may havedifferent designs. Transport of the material to be dried may beeffected, instead of gravitation, by means of a vibratting conveyor,conveyor belt, a scraping conveyor or by any other transporting means.In these cases the differential-analogue signal controls, e.g., theoperation of the drive. Heat-energy may be supplied beside the oilburner, e.g., by natural gas, heat-transfer oil, or the waste gas ofsome agricultural waste, coil- or peat-firing.

EXAMPLE

The process according to the invention will be described by means of apractical example:

The task consists of drying corn grains in a mass flow of 15 t/h with amoisture content of 32% to a reduced/dried moisture content of 14%. Intosection III about 120.000 kg/h drying agent is supplied, out of whichabout 40.000 kg/h comes from the cooling section. The expelled moistureremoved here by the drying agent amounts to about 8 g/kg, that meansthat together with the moisture quantity of 28.9 g/kg to be removed insections I and II, the specific vapour expulsion is not amounting tomore than 36.9 g/kg, accordingly the parameters of the discharged dryingagent are as follows: t=58° C., =27%, and the specific heat consumptionequals to 4,031.3 kJ/kg /963.0 kcal/kg/. Besides, the process ensures,whatever circumstances may prevail, i.e. even with changing moisturecontent of the fed material, a final product with uniform quality.

We claim:
 1. A process for drying agricultural and other granularproducts or produces having a surface and bound moisture content,comprising the steps of:(a) introducing said product into a drying areathat is subdivided into sections of preheating and intensive drying, asection for the removal of bound moisture from said product, and acooling section; (b) bringing a heated drying agent into contact withsaid product in the section for the removal of bound moisture from saidproduct; (c) directing the drying agent from the section for the removalof bound moisture to the sections for preheating and intensive drying ofsaid product; (d) venting the drying agent, laden with vapor from thedrying operation, from the drying area for further utilization or intothe open air; (e) measuring both the moisture content of the product tobe dried and fed into the drying area, and that of the product leavingthe cooling section of the drying area, and obtaining adifferential-analog signal therefrom; and (f) controlling the flow rateof the product to be dried and the temperature of the drying agent,individually or simultaneously, in response to said differential-analogsignal.
 2. The process of claim 1 wherein the drying agent isatmospherical air.
 3. Equipment for drying agricultural and granularproducts and produces having a surface and bound moisture content,through the use of a heat carrying drying agent, comprising:(a) a dryingarea provided with an inlet and a discharge opening for the product tobe dried, said drying area being subdivided into sections of preheatingand intensive drying, a section for the removal of bound moisture fromsaid product, and a cooling section; (b) a blowing duct having apressure fan connected thereto for the delivery of the drying agent tothe section of the drying area for the removal of bound moisture; (c) anexhaust duct having an exhaustor connected thereto for the removal ofthe drying agent; (d) a connecting duct situated in the drying area forthe transfer of the drying agent from the sectin for the removal ofbound moisture to the sections for preheating and intensive drying; (e)means for heating the drying agent, said means being positionedupstream, relative to the flow of the drying agent, of the pressure fanand communicatively engaged with said pressure fan via a heating duct;(f) a partition plate located between the sections for preheating andintensive drying and the section for the removal of bound moisture, saidpartition plate being adjustable for altering the physical limits ofsaid sections relative to each other; (g) at least one hygrometerpositioned at both the inlet and discharge opening of drying the areafor sensing the moisture content of said product; (h) transmitter meansfor sensing a signal received from the hygrometers which sense themoisture content of the product; and (i) control means for controllingthe rate of product discharge from the drying area and for adjusting thepartition plate in response to a signal received from said transmittermeans.
 4. The equipment defined by claim 3 wherein the control means forcontrolling the rate of product discharge from the drying area comprisesa slide valve, positioned at the discharge opening of said drying area,communicatively engaged with an operating cylinder having magneticvalves for actuating said slide valve in response to a signal receivedfrom the transmitter means.
 5. The equipment defined by claim 3 whereinthe means for heating the drying agent is positioned downstream,relative to the flow of the drying agent, of the pressure fan.
 6. Theequipment defined by claim 3 wherein the signal received from saidhygrometers are transmitted into the memory of a microprocessorcontroller.
 7. The equipment defined by claim 3 wherein the means forheating the drying agent comprises at least one oil burner connected incontrolled relationship with a temperature sensor positioned at theentrance to said blowing duct.
 8. The equipment defined by claim 7wherein the means for heating the drying agent comprises at least onegas burner.
 9. The equipment defined by claim 3 additionally comprisinga cooling fan communicatively engaged with the cooling section of thedrying area, the pressure side of the cooling fan being coupled to saidheating duct of said heating means.
 10. The equipment defined by claim 9wherein the pressure side of the cooling fan is coupled to the blowingduct.
 11. The equipment defined by claim 3 wherein the exhaust duct forremoval of the drying agent is connected to the sections for preheatingand intensive drying of the drying area via an output tray.